Senate Republicans have an urgent reason not to give up on repealing and replacing the Affordable Care Act: They don't want to break their promise to the GOP base. But the most recent polls suggest the base may not care as much as Republicans think.

The bottom line: A majority still supports the plan, but support has slipped, and there is some evidence that base Trump voters do not view repeal as a top priority — and many may not punish their representatives if they vote no.

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Data: Kaiser Family Foundation; Chart: Chris Canipe / Axios

The repeal push is motivated by several factors. Many Republicans in Congress want to undo whatever former President Barack Obama did. Some want to cap Medicaid spending. Others want to deliver a tax cut.

But the chief motivating factor is the desire to deliver on their campaign promise to the base to repeal "Obamacare," despite the problems with the legislation documented so thoroughly by the Congressional Budget Office. So it seems highly relevant to know where the base actually stands on the Republican replacement plan.

Consider these findings from our most recent June Kaiser Tracking Poll:

55% of President Trump's supporters still support the Republican replacement plan, but, as the chart shows, that number is down a notable 14 percentage points from May.

Trump supporters still want Trump and Republicans to keep working on a plan to repeal and replace the ACA (75%). But only 8% cite it as the most important priority for them, similar to the share of the public at large.

Less than half of Trump supporters — 44% — say they'd be less likely to support a member of Congress who votes against the bill.

Why it matters: Republicans have been single mindedly focused on their base because they know that only the most dedicated slivers of the electorate come out to vote in midterm elections. These data show that Trump voter intensity on the replacement plan has fallen. The polling data are similar if you look at Republican voters overall, rather than Trump voters.

If the intensity continues to fall, it could diminish this expected turnout. At a minimum, it could make repeal a less effective tool to whip up turnout in the base.

What we don't know: The vote, whenever it happens, could significantly affect the base in directions that are difficult to predict. So could the impact of the law itself if it passes. The CBO predicts that premiums and deductibles in the non-group market will begin to rise sharply and quickly, and Trump voters will be among the groups affected.

It is possible, therefore, that the immediate Republican victory lap that would come from passing repeal could end as Trump voters start to pay their health care bills.