A senator who is a leading GOP critic of President Donald Trump said the president has “unfortunately” redefined the Republican Party.

Arizona Republican Sen. Jeff Flake, in an appearance on "This Week" Sunday, was responding to ABC News Chief Anchor George Stephanopoulos, who noted that Trump has "consolidated his hold over the Republican Party, the Republican electorate."

"Has he redefined it?" Stephanopoulos asked.

"Unfortunately, yes," Flake replied. "When [former House Speaker] John Boehner said the other day, 'This is the president’s party,' he was speaking the truth."

Flake continued that the recent loss of Republican Rep. Mark Sanford in the GOP primary in South Carolina "clarified something if it wasn’t clarified before. You can’t, as a Republican these days, stand in ... opposition to some of the president’s policies or, or not condone his behavior and expect to win a Republican primary. That’s the reality and then we’re seeing that played out."

Sanford, who had been critical of Trump, lost in an upset in the GOP primary for his seat on June 12. The day of the election, Trump tweeted that voters should reject Sanford in favor of his challenger.

Flake also said on "This Week" that he hopes Trump is challenged in the 2020 primary for the Republican presidential nomination.

"I’ve said many times I hope that somebody does [run] in the Republican primary just to remind Republicans what it means to be conservative or Republican, that we believe in limited government, economic freedom, free trade, immigration," Flake said. "I hope that somebody does that."

The Arizona senator was also critical of Congress for failing to push for policies not supported by the president, such as on trade.

"We [in Congress] ought to more jealously guard our institutional prerogative," Flake said. "I think in this crisis we're in, I think the judiciary has stood up well. The press has stood up well in terms of institutions."

But the response of Congress "has been lacking," the senator said.