PORTLAND — Favorability ratings for President Barack Obama increased slightly in Maine since the fall, but his ratings in the state remain markedly lower than in the nation as a whole, according to a recent survey by Critical Insights.

The president’s favorability ratings in Maine increased in the spring to 47 percent from 43 percent in the fall, when it hit the lowest level during his administration, according to the Spring 2012 tracking survey by Portland-based Critical Insights.

Nationally, Obama’s favorability rate is 56 percent.

Favorability ratings for Mitt Romney, the presumptive Republican presidential nominee, are higher in the state than nationally.

In Maine, 51 percent have a favorable view of Romney, compared with 35 percent nationally. A quarter of Mainers polled are not sure about how they feel about the former Massachusetts governor.

The poll also showed that Maine Republican and Democratic candidates running for the U.S. Senate seat being vacated by Sen. Olympia Snowe have their work cut out for them.

The survey asked 600 residents to name any of the declared candidates.

Former Gov. Angus King, who’s an independent, was named by 46 percent of those surveyed. But 42 percent couldn’t name anybody.

Among Republican candidates, Secretary of State Charlie Summers had the top name recognition, named by 10 percent of those surveyed. State Sen. Cynthia Dill was tops among Democrats, named by 6 percent.

In a separate poll question, Snowe was given an 81 percent job approval rating.

The survey involved 600 telephone interviews of randomly selected voters between May 2-7. The survey has a margin of error of plus or minus 4 percentage points and a confidence level of 95 percent.