Three months before Election Day, it’s clear some senators may not return to Congress after the midterms — and that’s mostly good news for Republicans.

The GOP’s path to the Senate majority includes a mix of open seats and targeted Democratic incumbents. The two most vulnerable seats are in South Dakota and West Virginia, where Democratic senators are retiring. Republicans also have opportunities in open seats in Iowa and, to a lesser degree, Michigan.

But even if they are victorious in those states, the GOP must defeat at least two incumbents to reach the net six seats needed for control.

Luckily for Republicans, Democrats make up the vast majority of endangered senators seeking re-election. The GOP has a lengthy catalog of states where it has an opportunity to win, though there is a wide gap betweenthe No. 1 and No. 10 most vulnerable senators — who are ordered by most likely to lose.

Roll Call’s “10 Most Vulnerable Senators” list will be updated monthly ahead of the Nov. 4 elections. For now, here is where the incumbents stand:

1. Sen. John Walsh, D-Mont.
Running for the former seat of Democrat Max Baucus, Montana’s appointed senator was the most vulnerable incumbent prior to a New York Times report that he plagiarized a significant portion of a paper written for his master’s program at the Army War College seven years ago.

In a state that already leans Republican, Walsh is now firmly planted at the top of this list as he faces a top GOP recruit. Rep. Steve Daines raised more, spent more and had more in cash on hand at the end of the second fundraising quarter.

2. Sen. Mark Pryor, D-Ark.
Pryor would have made the top spot had Baucus not resigned to become ambassador to China, with Walsh appointed to take his place.

President Barack Obama took just 37 percent in Arkansas in 2012, and his approval rating there isn’t much better. Beyond that, Republicans landed a top recruit in freshman Rep. Tom Cotton, an Army veteran with two Harvard degrees and support from across his party’s broad spectrum.

Pryor’s rock-solid brand name could outweigh the reddening of Arkansas and the unfavorable national environment. He has a path, and because of that Pryor could conceivably drop on this list as the election nears. The question is, who surpasses him?

3. Sen. Mary L. Landrieu, D-La.
The Landrieu campaign is pushing to end the election on Nov. 4, when the three-term senator will face Rep. Bill Cassidy and a couple other Republicans in a jungle primary. But if she fails to eclipse 50 percent, it’s hard to predict what will happen in a one-on-one runoff.

Her fate in a Dec. 6 contest could depend on how the rest of the Senate races play out. If Democrats will retain control of the chamber no matter what happens in Louisiana, Landrieu’s odds improve. If Republicans have picked up enough seats to take the majority, Landrieu could be serving her final term.

4. Sen. Kay Hagan, D-N.C.
Hagan’s approval rating is underwater in a red-leaning state where Obama’s is too, and outside groups have already started spending in what is expected to be one of the most expensive races this cycle.

But state Speaker Thom Tillis, Hagan’s Republican opponent, is off to slow a start. After narrowly avoiding a runoff, he headed immediately into the legislative session, where he has been stuck for the past dozen weeks. He finished the second quarter with just $1.5 million in the bank; Hagan had almost six times that amount.

5. Sen. Mark Begich, D-Alaska
Begich’s 2008 defeat of legendary Republican Sen. Ted Stevens was by fewer than 4,000 votes and came just days after the incumbent’s conviction on federal corruption charges. Six years later, Begich will face one of a trio of prospective Republican challengers facing off in an Aug. 19 primary: Dan Sullivan, Mead Treadwell or Joe Miller.

After a pro-Begich super PAC has spent months slamming Sullivan on the airwaves, the campaign may see its best shot at the Ohio-born, former commissioner of the state Natural Resources department. But Begich has already been hit considerably on TV as well, and there will be plenty more outside spending on the state’s cheap airwaves in the final weeks.

6. Sen. Mark Udall, D-Colo.
Udall was supposed to have a fairly easy coast to re-election. Then Rep. Cory Gardner made a last minute decision to enter the race, and now Udall has a fight on his hands. Polls put the two in a statistical tie.

With the entry of a strong Republican candidate came the flood of outside money to oust the Democratic senator and to protect him. Udall’s campaign went negative against Gardner in their very first ad. In Colorado, a truly purple state with libertarian tendencies, the negativity is probably there for the duration.

7. Sen. Mitch McConnell, R-Ky.
The lone Republican on this list, McConnell is again involved in an expensive and close race, as he was in 2008. Kentucky Secretary of State Alison Lundergan Grimes is one of the top fundraisers in the country, largely because she’s challenging the Senate minority leader.

McConnell has eschewed assistance from the National Republican Senatorial Committee, which he wants to focus on picking up a net six seats, winning Senate control and making him majority leader. He’ll first need to get past Grimes in a race that promises to be close but in which Republicans hold the advantage.

8. Jeanne Shaheen, D-N.H.
Shaheen drew a strong opponent when former Massachusetts Sen. Scott P. Brown moved one state over and decided to run for Senate. But Brown has struggled to translate his much-lauded likeability to his newly adopted home. Shaheen, meanwhile, remains popular, and she has a healthy lead over Brown in most polls and in fundraising.

New Hampshire Republicans say Brown’s fortunes depend on the national climate: If it is a powerful wave year for Republicans, he will likely win. If it is not, Shaheen is headed back to the Senate.

9. Al Franken, D-Minn.
Franken is sitting on $5 million in a state Obama won twice, giving him a decided edge in his race for re-election. But Franken drew a strong Republican opponent in Mike McFadden, a businessman with fundraising chops to make it competitive, and the personal resources to help his campaign along. Even Minnesota Democrats acknowledge it will likely be tight. But, as of right now, Franken is probably headed back to the Senate.

10. Jeff Merkley, D-Ore.
Merkley is likely headed back to the Senate. Recent polls put him about 15 points ahead of Republican Monica Wehby, and he has a hefty cash advantage.

Obama won Oregon by double digits twice. But just six years ago, Merkley was the underdog heading into the final three months of his race against Republican Sen. Gordon Smith. Merkley won, but narrowly, toppling Smith by 3 points.

Oregon has also seen the uglier side of the new health care law. The state scrapped its state-run exchange in April — and Wehby, a doctor, could capitalize on such a climate.

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