In a statement, he said he was stepping down "with a heavy heart," having given his constituents "my all over the last six years." "But the constant questions over the last six weeks have proven a great distraction that has made it too difficult for me to serve the people of the 18th District with the high standards that they deserve and which I have set for myself," he added.

The coverage gains have delivered the largest drop in the uninsured rate in four decades, bringing that rate down to 13.2% by the end of the first quarter of 2015, the Department of Health and Human Services announced Monday. That's down from about 20% before the health insurance marketplaces launched in late 2013.