In the midst of the earliest presidential campaign season in history, Senators are now wrestling with one of the tightest Congressional calendars in memory, and many fear they are entering a new era where lawmakers have only six months per Congress to enact any weighty bipartisan reforms.

No longer, Senators in both parties acknowledged, do they have the luxury of making use of the entire off-year of a presidential cycle, and perhaps a brief period of the election year itself, to work through complex and controversial issues. As a result, Democrats and Republicans alike say they are under heightened pressure to pass legislation at a frenetic pace before they lose Senate presidential hopefuls to the trail entirely, the early White House frontrunners set their parties’ priorities and the public loses focus on the Congressional agenda.

“For all intents and purposes, the window to legislate is getting narrower and narrower all the time,” said Sen. John Thune (R-S.D.). “And that’s a problem.”

“I don’t like it,” agreed Sen. Ken Salazar (D-Colo.). “Presidential politics is creeping into the most fundamental decisions we make here, and it’s not useful in terms of our ability to legislate on behalf of the people. It has a distractive effect on what we are doing here in Congress.”

Already, Senators are feeling the heat of the 2008 White House campaign, most particularly on immigration reform, which both parties have acknowledged must get done before the outside pressures from 2008 presidential politics take hold. Majority Leader Harry Reid (D-Nev.) has said for months that Senate passage of a bill must be achieved before lawmakers return home for the August recess, and on Tuesday he reiterated that it may be now or never for any serious immigration overhaul.

But immigration is not the only issue under the gun on the ever-tightening Congressional calendar.

Chances to approve bipartisan health care and energy reforms, as well as agree on 12 spending bills, are all being viewed against the backdrop of the August break, which many Senators worry will serve as the deadline for substantive deal-making in the 110th Congress. Some issues aren’t even making that late summer cutoff, including an overhaul of the nation’s entitlement programs for which neither party appears to have the political appetite.

Sen. Richard Burr (R-N.C.), first elected to the Senate in 2004, said while the presidential election always has swayed Congressional activity, it is doing so even more in the 110th Congress because of the front-loaded primary calendar. Congress used to be able to escape the influences of the campaign until at least the election year itself, but no longer, he said.

“The window of opportunity here is the next 60 days,” Burr said. “It doesn’t lend itself to getting a lot done. Still, an urgency to do something comprehensive [on] health care exists, an urgency to get something comprehensive done on energy reform exists. Now, the timing has a consequence to not doing them.”

Reid, clearly not oblivious to the challenges before him as the Majority Leader, on Tuesday laid out a laundry list of priorities for the next seven weeks before the August recess, beginning with completing immigration reform, dealing with gas prices and passing a package of energy reforms, continuing to debate the Iraq War, enacting a farm bill, expanding the children’s health insurance program, approving a series of conference reports and laying out spending plans for scores of government agencies.

“Look at the calendar,” Reid instructed reporters as he explained why time is ticking on Senate passage this week of an immigration plan.

Democratic and Republican Senators alike said several 2008 factors are shaping the atmosphere for the session beyond the early primary calendar. At least six Senators — four Democrats and two Republicans — are running for their parties’ nominations and are splitting their focus between the Senate and the stump. And much of what those candidates say and do can have a direct impact on Congressional business and vice versa.

What’s more, both Democrats and Republicans are hoping their respective party nominees will help bolster their chances to make gains in the House and Senate in 2008.

“I don’t think there’s a lot we can do about it,” said Senate Minority Whip Trent Lott (R-Miss.). “It’s a fact that the presidential process is earlier than ever.”

Sen. Debbie Stabenow (D-Mich.) conceded the early primary schedule appears to be here to stay, but at least one factor — a crowded field of candidates seeking the White House with no sitting president or vice president running — may be limited to affecting the 110th Congressional calendar. “We won’t know until the next election whether this is a trend or not. My guess is, next time, if there is an incumbent running this won’t be the case.”

Republicans are hoping the current Congressional dynamic will bear dividends for their party since Democrats will face the burden of proving to voters why they should be kept in charge of Congress. To do that, Democrats must show they have racked up accomplishments in the House and Senate, something they have yet to do, GOP Senators argued.

“It will” hurt them, Lott said. “Just like it fell on us last year. Basically we did nothing. Democrats are headed down that same trail. Their whole agenda is Iraq. They are putting themselves in as terrible a position as possible.”

“Obviously, they are in charge and bear a greater share of the responsibility,” Thune said.

And while Democrats don’t discount the presidential pressures before them, they are unwilling to accede to Republican charges that they are lax in legislating. From passing a minimum-wage increase, increasing federal stem-cell research funding, putting together a budget blueprint and keeping the heat on the Bush administration to change the direction of the war, they insist they are proving their worth to the electorate.

“I agree it gets harder because of the caucuses and the primaries, but we are going to get a lot of things done this year,” argued Sen. Byron Dorgan (D-N.D.). “And we’re not going to give up on getting things done next year.”

“It is what it is,” Salazar said. “We’ll work through it.”