But a comparison of her past goals and present ambitions makes clear that Ms. Stefanik has morphed from optimist to operator, choosing short-term power over the long-term health of her party. When I interviewed Ms. Stefanik in 2018 and 2019, she seemed to understand that the Republican Party was in trouble with young people. “The G.O.P. needs to prioritize reaching out to younger voters,” she told me. “Millennials bring a sense of bipartisanship and really rolling up our sleeves and getting things done.” Now she has tied her political career to the man who has perhaps done more than any other Republican to drive young voters away from her party, resulting in surging youth turnout for Democrats in the 2018 and 2020 elections. Ms. Stefanik’s rise — and her colleagues’ fall — is not just a parable of Trumpism. It’s a broader omen for a party struggling to reach a 21st-century electorate. She ascended by embracing a movement that is all about relitigating the past rather than welcoming the future. Now she and other new Trump loyalists in Congress are caught between their party and their generations, stuck between their immediate ambitions and the long-term trends. The G.O.P. has embraced a political form of youth sacrifice, immolating their hopes for young supporters in order to appease an ancient, vengeful power.