With under three weeks remaining until the Republican primary, the battle to define state House Speaker Thom Tillis (R) is on in North Carolina.

Speaker of the North Carolina House of Representatives, Thom Tillis (R). (AP Photo/The Wilson Times, Brad Coville)

Just this week, a Democratic super PAC, the campaign of Sen. Kay Hagan (D) and Tillis's campaign went up with commercials trying to define the Republican for voters.

Why? Because, like much of politics, timing is everything in this case. If Tillis doesn't clear the 40 percent mark in the May 6 primary, he will be forced into a runoff against another Republican on July 15. His opponents include physician Greg Brannon, Baptist minister Mark Harris -- both running to Tillis's right -- and a handful of other candidates.

So it makes sense that Democrats would try to pile on with dual-purpose ads that could both hurt Tillis among moderates ahead of the general election and also raise eyebrows among conservatives who vote in Republican primaries.

TV ad from the Democratic Senate Majority PAC takes Tillis to task for former aides who were involved in extramarital affairs with lobbyists -- a line of attack that could resonate with evangelical Christians who vote in GOP primaries. Hagan's campaign released its own radio ad which mentions the affairs and also claims Tillis wants it "both ways" on Obamacare, portraying him as supportive of certain elements of the law.

Hagan spokeswoman Sadie Weiner said the ad is not about the primary. Rather, the purpose of the commercial is "to set the record straight that Tillis is trying to have it both ways." Still, suggesting Tillis is any way supportive of the health-care law could hurt his support on the right, regardless of intent.

Tillis's campaign fired back with its own TV ad accusing Democrats of trying to reduce his standing in the primary because they fear him. "[Senate Majority Leader Harry] Reid and Hagan know they cannot win against Thom Tillis in November, but North Carolina conservatives won't be fooled by misleading attack ads by DC liberals," said Tillis spokesman Jordan Shaw.

Here's the bottom line: Is Tillis going to be the Republican nominee? Almost certainly. The question is whether it happens in May or July. Democrats are already running against Tillis in anticipation of his nomination. But the timing of their ads could affect when his nomination happens. A July nomination after another two months of costly GOP infighting would be welcome news for Hagan.

Below we rank the 11 Senate races most likely to switch parties this November. The number one race is considered the most likely to switch parties.

To the Line!

10. Colorado (Democratic-controlled): Rep. Cory Gardner (R) had a big fundraising month in March, pulling in over $1.2 million. It's money he'll need to fend off attacks that are already landing. The League of Conservation Voters just hit the airwaves with about a $1 million ad buy bashing Gardner for taking money from the oil and gas industries. Meanwhile, Sen. Mark Udall (D) had a big fundraising quarter, pulling in more than $2 million. (Previous ranking: 10)

9. Michigan (D)/Georgia (Republican-controlled): Former secretary of state Terri Lynn Land (R) and Rep. Gary Peters (D) had very comparable first quarter fundraising totals in Michigan, which is good news for the underdog Land. In Georgia, Democrat Michelle Nunn had another huge quarter ($2.4 million) and save for Rep. Jack Kingston and businessman David Perdue, the rest of the Republican field is cash-strapped. If the election were held today, Kingston and Perdue would probably make it to a runoff, since Rep. Phil Gingrey has come under attack from a super PAC and the fundraising woes of Rep. Paul Broun and former secretary of state Karen Handel have prevented them from making big moves. But the election is not today, it's May 20. Plenty can change. (Previous rankings: 9)

8. Kentucky (R): Secretary of State Alison Lundergan Grimes (D) raised a very impressive $2.7 million, outpacing Senate Minority Leader Mitch McConnell in the first quarter. That said, McConnell won't be at a cash shortage this year. Before he can seriously begin to engage Grimes, McConnell must defeat businessman Matt Bevin. With the May 20 primary about a month away, Bevin hasn't gained any real momentum, and McConnell looks like a safe bet to advance. (Previous ranking: 8)


7. Alaska (D): A recent health-care ad from a pro-Sen. Mark Begich (D) super PAC was one of the most eye-catching of this election cycle so far. The ad makes a deeply personal pitch for Begich's support for the health-care law. It features a woman who says she survived cancer but was later denied coverage because of her preexisting condition. She says she is covered now, thanks to the health-care law. The ad is striking because most Democrats are running defensive messages on Obamacare. Meanwhile, Dan Sullivan (R) had another good quarter, outraising Begich once again. (Previous ranking: 7)

6. North Carolina (D): Hagan has been more negatively impacted by the early ad blitz from Americans for Prosperity than any of her red state Democratic counterparts. It hasn't hurt her fundraising, though: She raised $2.8 million between January and March -- more than double what Tillis brought in. (Previous ranking: 6)

5. Arkansas (D): Don't call Sen. Mark Pryor (D) 2014's Blanche Lincoln. Lincoln was the last Democratic senator to run for reelection in Arkansas, and she lost by 20 points. Thanks to a robust coordinated campaign and a data operation churning along at an accelerated pace, Democrats feel better about this race than you might think given the year and the state. Still, it's an uphill climb. Mitt Romney won here by double-digits, and Rep. Tom Cotton (R) is one of the GOP's most prized recruits. He outraised Pryor last quarter. (Previous ranking: 5)

4. Louisiana (D): Sen. Mary Landrieu (D) is pushing back against a blitz of conservative attack ads with a commercial underscoring her Washington ties. It's actually a smart play. As the newly installed chair of the Senate Energy and Natural Resources Committee, Landrieu wields a lot of power on an issue that will resonate back home since the state's economy is centered in the oil and gas industries. The leading Republican running against Landrieu, Rep. Bill Cassidy (R), had his best fundraising quarter yet. Landrieu still outpaced him. (Previous ranking: 4)

3. Montana (D): After all the hype surrounding Democrat John Walsh's appointment to the Senate, we thought he would have at least cracked $1 million for the first quarter, given his new position of power. He came close but didn't. Rep. Steve Daines (D), his leading Republican opponent, did. (Previous ranking: 3)

2. West Virginia (D): Secretary of State Natalie Tennant (D) had a good fundraising quarter. She nearly matched the total of front-runing Rep. Shelley Moore Capito (R). But Capito is still the candidate to beat here. If your name isn't Joe Manchin or Earl Ray  Tomblin, it's not an ideal time to be a Democrat in West Virginia. The Obama administration is deeply unpopular there, and while Democrats have a voter registration advantage, a considerable chunk of those "Democrats" have been consistently voting for the Republican candidate at the federal level in recent years and simply never bothered to switch their party affiliation. (Previous ranking: 2)

1. South Dakota (D): Retiring Democratic Sen. Tim Johnson's seat is still former governor Mike Rounds' (R) to lose. The Republican had a good fundraising first quarter, and Democrat Rick Weiland is still trying to make a name for himself. (Previous ranking: 1)