At the dawn of the election year, Senate Republicans feel a renewed sense of optimism in their fight to net the six seats necessary to win the majority in 2014.

The stumbling rollout of Obamacare has given the GOP hope it can loosen the Democrats’ grip on the majority by ousting its four most vulnerable incumbents.

Still, several factors could influence how things appear in the final months of President Barack Obama’s second midterm cycle. And Democrats have proved in recent years they know how to win tough Senate races.

Will the Obamacare implementation improve? Will the economy rebound? How big of a fight will Republicans put up over the debt ceiling? And how about the GOP’s pesky primaries?

These unknowns are why optimism — but not overconfidence — is the overriding feeling among the Senate GOP.

“I’m more bullish about the GOP’s chances in the Senate now than I have been at any point this cycle,” said Dan Judy, a pollster at GOP firm North Star Opinion Research. “The shutdown seems like a distant memory and the Ryan/Murray deal went a long way to settling things down so that our guys can really focus on the midterms.”

Meanwhile, Democratic pollster Celinda Lake cited GOP primary issues, well-prepared candidates and demographic changes among the reason for Democrats’ positive outlook, even as Obama’s approval ratings hover in the low 40s.

The GOP’s own unpopularity “gives these senators the ability to individually define their opponents, and Senate races have the kind of money necessary to individualize,” Lake said.

Here is a rundown of the competitive Senate playing field as it stands today, with Rothenberg Political Report/Roll Call race ratings:

Most Vulnerable Incumbents
Sen. Mark Begich, D-Alaska

The late Aug. 19 primary gives the eventual GOP nominee less than three months to focus solely on Begich. Former Natural Resources Commissioner Dan Sullivan is outraising his opponents, and a super PAC backing him just launched its first ads against Begich.

Rating: Tossup/Tilt Democrat

Sen. Mark Pryor, D-Ark.

Democrats are defending Pryor in part by defining Republican Rep. Tom Cotton as a “reckless and irresponsible” alternative. The state is trending Republican, and Cotton has the support of both the national party and conservative outside groups.

Rating: Tossup/Tilt Republican

Sen. Mitch McConnell, R-Ky.

The Senate minority leader faces opponents on his right and left. Before taking on Secretary of State Alison Lundergan Grimes, McConnell must first dispense with Republican businessman Matt Bevin, who is supported by conservative outside groups.

Rating: Lean Republican

Sen. Mary L. Landrieu, D-La.

Landrieu has never won an election with more than 52 percent in the Bayou State, which voted 58 percent for Mitt Romney in 2012. This year she’s facing a top Republican recruit in Rep. Bill Cassidy. They’ll face off in the November jungle primary with Republican Rob Manness, who is backed by conservative outside groups, before a likely runoff.

Rating: Pure Tossup

Sen. Kay Hagan, D-N.C.

Despite a crowded GOP primary that could weigh down the eventual nominee, Republicans are at least aided here by its early date (May 6). Still, the incumbent has been preparing for this race for nearly a year already, and Republicans could end up with a weaker candidate than they hoped. Speaker Thom Tillis is the party’s top recruit there, but he faces tea party favorite Greg Brannon and pastor Mark Harris, among others.

Rating: Tossup/Tilt Democrat

Top-Tier Open Seats
Georgia: Retiring Republican Saxby Chambliss

This primary could have the greatest effect on the competitiveness of the general of any Senate race this year. Reps. Paul Broun and Phil Gingrey are likely to coalesce conservative primary voters, who could propel one of them to the top of a crowded field. Former Points of Light Foundation head Michelle Nunn, a Democrat, is raising serious cash and awaiting her opponent.

Rating: Republican Favored

Montana: Retiring Democrat Max Baucus

What this race looks like in November will depend on how quickly Baucus is confirmed as ambassador to China and whom the Democratic governor appoints to replace him. Republican Rep. Steve Daines and Democratic Lt. Gov. John Walsh will likely face off in the general in this top pickup opportunity for the GOP.

Rating: Tossup/Tilt Republican

South Dakota: Retiring Democrat Tim Johnson

Democrats missed out on their top two potential recruits and now appear likely to lose the seat in November. Former Gov. Mike Rounds is the favorite among the Republicans, but he faces a crowded primary before likely meeting former Democratic congressional aide Rick Weiland in the general.

Rating: Republican Favored

West Virginia: Retiring Democrat Jay Rockefeller

GOP Rep. Shelley Moore Capito entered the race in November 2012. She’ll likely face Secretary of State Natalie Tennant, a Democrat, in another top GOP pickup opportunity.

Rating: Lean Republican

Second-Tier Open Seats
Iowa: Retiring Democrat Tom Harkin

Democratic Rep. Bruce Braley may have to wait out both a crowded Republican primary and convention before knowing whom he’ll face in November.

Rating: Lean Democrat

Michigan: Retiring Democrat Carl Levin

Democratic Rep. Gary Peters is facing Republican former Secretary of State Terri Lynn Land. Republicans have won only a single Senate race in this state since 1972.

Rating: Democrat Favored

Democrats in Other Competitive States
Sen. Jeanne Shaheen, New Hampshire

Shaheen’s greatest challenge could come from former Sen. Scott P. Brown, R-Mass. Brown hasn’t announced his plans yet, but winning a Senate race in two different states is nearly unprecedented.

Rating: Democrat Favored

Sen. Mark Warner, Virginia

It’s hard to find an incumbent from a purple or red state in a better position than Warner. And it remains to be seen what former Republican National Committee Chairman Ed Gillespie’s path to victory will be.

Rating: Safe Democrat