CHICAGO (AP) — Republican U.S. Sen. Mark Kirk says Donald Trump’s comments about a U.S. federal judge of Mexican heritage are un-American and he cannot support the presumptive presidential nominee.

Tuesday’s statement is a reversal for Kirk, one of the most endangered Republican incumbents, who has said previously he would support Trump.

It comes as Democrats, including Senate rival U.S. Rep. Tammy Duckworth, have stepped up their criticism of Kirk for not denouncing Trump.

In his statement, Kirk says Trump’s “belief that an American-born judge of Mexican descent is incapable of fairly presiding over his case is not only dead wrong, it is un-American.”

Kirk said he was hoping the rhetoric would tone down.

Trump issued a statement saying his attacks on the judge handling the Trump University case have been “misconstrued.”