Here's the latest evidence that Democrats may not trounce the GOP in the 2018 midterms

Four competitive races in the House of Representatives have shifted, and are more likely to be won by Republicans than they once were, the Cook Political Report predicted on Tuesday, in the latest sign that Democrats' midterm advantage is disappearing.

Democrats are still solidly favored nationally, but the landscape surrounding high-stakes primaries is rapidly changing. And while Democrats still lead Republicans by four percentage points in Real Clear Politics' congressional generic ballot, that lead is a fraction of the 13-point advantage they held in December.

California's 39th and 49th districts have both shifted from "lean Democratic" to "toss up," the Cook Political Report says, as a crowded field of Democratic candidates threaten to splice the share of votes. The open ballot policy in California means it's possible for two Republicans and no Democrats to make it through the primaries to the general election.

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In South Carolina's 5th district, ratings shifted from "likely Republican" to "solid Republican." South Carolina's race has been affected by recent allegations that Democrat Archie Parnell assaulted his ex-wife in the 1970s — Parnell has so far opted to stay in the race even though top Democrats and his own staffers have renounced support.

Nebraska's 2nd district went from "toss up" to "lean Republican." After progressive candidate Kara Eastman beat out the Democratic Party's moderate pick in Nebraska's primary, analysts say Eastman may be too liberal for the district as she goes up against incumbent Rep. Don Bacon (R).

Read more analysis at the Cook Political Report.