Just as Mitt Romney is aiming to add to his delegate lead in the Republican primary race with strong showings in Arizona and Michigan, his popularity among conservatives nationally is fading, according to a new Washington Post-ABC News poll.

Only 38 percent of “very conservative” Americans now express favorable views of the former Massachusetts governor. That’s his lowest mark of the campaign among staunch conservatives – a group he’s also struggled with in previous contests – and down 16 percentage points over the past two weeks.

Romney’s main challenger for these votes in Tuesday’s primaries is Rick Santorum, who has held up far better among the most conservative across the country, with three times as many expressing favorable than unfavorable views.

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Switching focus to conservative Republicans and independents – including the “somewhat conservative,” Romney’s recent slippage is evident among both men and women and among those with college degree and those without.

Among men, nearly as many conservatives now express unfavorable views as favorable ones of Romney. Only a week ago he was about 2 to 1 positive.

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But the biggest volatility in Romney’s ratings has been among conservative Republicans and independents with household incomes under $50,000. Here, Romney’s support has bumped up and down, with 37 percent now holding positive views, and just as many, 40 percent, negative ones. By contrast, in this group, Santorum continues to be well reviewed.

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This three-item favorability poll also includes an update on President Obama, who stays just above 50 percent on this most basic measure of popularity. He is at 51 percent, with 45 percent of all Americans expressing unfavorable views.

Political independents split about evenly on the president and Santorum alike, with the major difference that more than a third expresses “no opinion” about the former Pennsylvania senator. Romney remains underwater among independents.

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The poll was conducted Feb. 22 to 26 among a random national sample of 1,011 adults. Results from the full survey have a margin of sampling error of plus or minus 3.5 percentage points.

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