Rick Santorum has surged to match Mitt Romney among Republican voters nationally, according to a poll released Monday by the Pew Research Center, conducted after Santorum’s sweep of last week’s contests in Colorado, Missouri and Minnesota. Santorum and Romney win 30 and 28 percent support, respectively; Gingrich wins 17 percent and Paul wins 12. Santorum holds double-digit leads among conservative voters, white evangelical Protestants and tea party voters.

Core Republican groups are increasingly skeptical of Romney’s conservative credentials, especially those who agree with the tea party movement. Among this group, over two in three now say Romney isn’t a strong conservative, up dramatically from about four in 10 who said this in last November. Half of all Republican-leaning voters doubt Romney’s conservative bona fides, up from one third in the fall.

Nevertheless, Romney’s aura of electability hasn’t faded, even among his weakest voting groups. Nearly three quarters of tea party Republicans say Romney is well-qualified to be president and two thirds view him as honest. About half of GOP voters and tea party sympathizers alike say Romney understands their problems, somewhat less positive marks than in November.

Romney’s overall image took a hit in January, according to a Washington Post-ABC News poll, with more Republicans, Democrats and independents seeing him as “unfavorable.”

More from the Post polling team

Sign up for Post polling e-mails

Follow Post polling on Twitter

Like Post Politics on Facebook