Multiple indicators, including generic ballot polls, President Trump's approval ratings and recent special election results, point to midterm danger for Republicans. But without robust race-by-race polling, it's trickier to predict individual races six months out. Are Democrats the favorites to pick up the 23 seats they need for a majority? Yes, but it's still not certain which races will materialize for Democrats and which won't.

Our latest ratings point to 56 vulnerable GOP-held seats, versus six vulnerable Democratic seats. Of the 56 GOP seats at risk, 15 are open seats created by retirements. Even if Democrats were to pick up two-thirds of those seats, they would still need to hold all their own seats and defeat 13 Republican incumbents to reach the magic number of 218. Today, there are 18 GOP incumbents in our Toss Up column.

That Toss Up list is likely to grow as the cycle progresses. Out of the 65 GOP incumbents rated as less than "Solid," 49 were first elected in 2010 or after, meaning more than three quarters have never had to face this kind of political climate before. And, Democrats have a donor enthusiasm edge: in the first quarter of 2018, at least 43 sitting Republicans were out-raised by at least one Democratic opponent.

In 2010, House Democrats suffered a backlash against their votes for two polarizing pieces of legislation: cap and trade (which died in the Senate) and the Affordable Care Act (which passed). 2018 may be a mirror image: House Republicans must defend their votes for the AHCA (which polled far worse than Obamacare and died in the Senate) and the tax bill. A new Gallup poll found voters still disapprove of the tax bill, 52 percent to 39 percent, four months after passage.

Armed with fresh FEC data, we have created a table listing seven "risk factors" to gauge Republican incumbents' political health and readiness for a wave election. In the past, those incumbents with a high number of risk factors have typically been the ripest targets, while those with fewer risk factors could still be vulnerable but may be better able to withstand a hostile political environment.

The seven risk factors are:

Sits in a district with a Cook PVI score of R+5 or less Republican. Sits in a district that Hillary Clinton carried in 2016. Received 55 percent of the vote or less in the 2016 election (or a 2017 special election). Voted in favor of the American Health Care Act in the May 4 roll call vote. Voted in favor of the Tax Cuts and Jobs Act in the December 19 roll call vote. Raised less money than at least one Democratic opponent in the first quarter of 2018. Has a Democratic opponent with at least $200,000 in cash on hand as of March 31.

Only one incumbent, Rep. Steve Knight (CA-25), has all seven risk factors. Eight incumbents have six risk factors, 23 incumbents have five, 23 incumbents have four and 32 have three. This is not a hard and fast list, and over the next quarter, many incumbents will add or subtract factors based on their own and their opponents' progress.

Risk Factors

Seven Risk Factors

Six Risk Factors

Five Risk Factors

Four Risk Factors

Three Risk Factors

Web Editor Ally Flinn contributed to this report