Both parties will try to channel the populist zeal sweeping the nation into election victories this November.

Republicans are running against government encroachment, and Democrats are ratcheting up their rhetoric against Wall Street financial institutions. But these themes will be tested in races held months earlier when several incumbents face rare primary challenges.

The near-insuperable odds of unseating a sitting Member of Congress in a primary election hasn’t deterred dozens of challengers from undertaking the effort. Most will fail, but the reward can be worth the risk.

Some of the Democratic Party’s oldest and longest-serving Members are being challenged by young upstarts who say Congress needs new blood, while others are being targeted for the way they have conducted themselves in office.

On the Republican side, this year’s most notable primary races involving incumbents are less personal and more policy-oriented. Many GOP challengers are affiliating themselves with the activist “tea party” movement and portraying themselves as more conservative than the incumbents they hope to unseat.

“This is the populist wing of the party coming to the fore,” said Gary C. Jacobson, a political scientist at the University of California at San Diego.

Robert G. Boatright, a Clark University political scientist who has studied Congressional primaries, said a party tends to have an above-average number of primary challenges in years of political opportunity. That was the case in 1974, when many young liberals took on more conservative senior Democrats ahead of the party’s big gains that November, and in 1994, when Republicans last won control of the House from Democrats.

“It’s looking like it’s going to be a year the Republicans pick up a lot of seats,” Boatright said. “The way these trends have worked out the past few decades, I think there will be a lot of Republicans who face primary challenges, as well. There are a lot of conservatives who are very fired up.”

In Democratic primaries, the most notable challenges involve senior incumbents who are nearing the end of their political careers. A case in point is Rep. Paul Kanjorski (Pa.), 72, whose challenger, county commissioner Corey O’Brien, is half the Congressman’s age. O’Brien will try to link Kanjorski , a senior member of the Financial Services Committee, to the ongoing economic problems by arguing that he is more focused on Wall Street than on Main Street.

“The people really want a significant change in their leadership, and they want a change that is focused on jobs and job creation,” O’Brien said in a recent interview with Roll Call.

Rep. Charlie Rangel (N.Y.), the 79-year-old chairman of the Ways and Means Committee, faces intraparty opposition from Vincent Morgan, a former campaign director, as well as from state Rep. Adam Clayton Powell IV, the namesake son of Rangel’s House predecessor and the same Democrat who challenged Rangel in a 1994 primary. Rangel has had to deal with investigations into his personal finances and fundraising.

Rep. Alan Mollohan (W.Va.), a senior member of the Appropriations Committee, is facing opposition in the primary election for only the third time in 14 re-election campaigns. Mollohan has faced questions about whether he has had too close ties with some nonprofit groups that were recipients of federal funds that the Congressman earmarked. The Justice Department recently ended its investigation into Mollohan.

State Sen. Mike Oliverio, who is challenging Mollohan, told Roll Call in a recent interview that he is “not interested in tearing Alan Mollohan down.” But he hinted at a line of attack against the Congressman when he said in his campaign announcement that Democrats “should be helping those least fortunate, not funneling money to pet projects.”

Sen. Arlen Specter (Pa.), who turned 80 on Friday, faces opposition in the Democratic primary from Rep. Joe Sestak, who is more than 20 years his junior. Specter has sided overwhelmingly with Democratic leaders since switching parties last spring and is even trying to run to Sestak’s left on some issues. Sestak portrays himself as the more loyal Democrat, saying Specter switched parties out of political opportunism. With the White House and the rest of the Democratic establishment backing Specter, Sestak says candidates should not be anointed in primaries.

In the only other competitive Democratic primary challenge to a sitting senator, former Colorado Speaker Andrew Romanoff is taking an anti-Wall Street tack as he takes on appointed Sen. Michael Bennet. No real differences on policy have yet emerged in that race, though one interesting subtext is that Romanoff is challenging Bennet after Gov. Bill Ritter (D) passed him over for the Senate vacancy.

In the Republican Party, conservative challengers are allying themselves with the burgeoning “tea party” movement and criticizing GOP incumbents as insufficiently conservative.

One of the newest and highest-profile GOP primaries to emerge is in Arizona, where former Rep. J.D. Hayworth is preparing a challenge to Sen. John McCain, the 2008 Republican presidential nominee. Hayworth plans to portray McCain, who long cultivated an image as a political maverick, as too moderate for the Arizona GOP electorate. McCain has sided more frequently with his party after losing the presidential election to Barack Obama.

Sen. Bob Bennett (Utah), who’s running for a fourth term, also has to be mindful of a serious challenge on his right flank. Bennett faces several opponents, including former federal prosecutor Mike Lee and wealthy businessman Tim Bridgewater, and the conservative political organization Club for Growth has criticized Bennett for voting for the $700 billion bailout to stabilize the financial markets.

In Texas, 11 of the 20 Republican members of the House delegation drew opposition in the March 2 primary, the most in at least 30 years. They include Reps. Ralph Hall, at 86 the oldest House Member, and Ron Paul, a libertarian-leaning Republican who ran for president in 2008. Many of the primary challengers tout tea party connections.

Most of the states and districts where Republican incumbents face challenges in primary elections are so strongly Republican-leaning that Democrats are unlikely to be competitive. Still, Jacobson said it could be problematic for Republicans if a conservative tea party candidate were to unseat a centrist GOP officeholder in a politically competitive district in which the Democrats would have a better chance against the conservative candidate.

“It’s a problem for the Republican Party in that there are not enough conservatives to win a majority,” Jacobson said.

In two other notable GOP primaries, challengers aren’t yielding to incumbents who weren’t expected to enter this year’s contests.

Rep. Parker Griffith (Ala.) is running for re-election with the backing of GOP officials who hailed his switch from the Democratic Party in December, though he faces opposition in the Republican primary from Mo Brooks, a county commissioner, and Les Phillip, a businessman and Navy veteran. Brooks and Phillip have been campaigning for months and refuse to yield to Griffith in the primary.

Rep. Jim Gerlach (Pa.) is on firmer ground in his primary after ending his long-planned gubernatorial campaign. But Steve Welch, a wealthy businessman, has said he is staying in the GOP race.

At least two senior Republicans are drawing primary challenges after demonstrating weakness in 2008 primaries. Rep. Don Young (Alaska), who was nearly defeated for renomination in 2008 by then-Lt. Gov. Sean Parnell, will be challenged by former state legislator Andrew Halcro.

Rep. Dan Burton (Ind.) was renominated in 2008 with just 52 percent of the vote in the 5th district in and around Indianapolis. In that election, Burton defeated John McGoff, a former county coroner who is one of at least six primary challengers this year. While the large field opposing Burton highlights the Congressman’s vulnerability, Burton stands to benefit from divided opposition and could win with a plurality of the vote.