MEREDITH, N.H. — Someone needs to whisper this into Beto O’Rourke’s ear before the next Democratic debate: “Step up or step away.”

The twin debates in Detroit were a vital moment for all 20 candidates who have qualified for the televised events; and the last proving ground before the party, through more stringent debate standards for the September confrontations, begins the process of winnowing the field. But perhaps no candidate has more at stake than the former Texas congressman, a onetime phenom now fading fast.

Mr. O’Rourke occupies a unique position among the Democratic contenders. He’s not in the first tier of candidates. But his fundraising power and potential make him a possible member of that top level, if only he can have the sort of breakthrough moment that Sen. Kamala Harris of California had late last month when she challenged former Vice President Joseph R. Biden Jr. on his civil rights record.

“There’s a second tier of candidates with real talent,” says Thomas A. Devine, chief strategist for the 2016 surge of Sen. Bernie Sanders of Vermont and senior adviser and strategist for 2004 Democratic nominee John F. Kerry. “Beto is one of them. He’s a very interesting character, but he hasn’t made the connection yet.”

Indeed, Mr. O’Rourke is teetering between the two tiers, and his performance here at a stunning home on Lake Winnipesaukee’s Cummings Cove, just off a narrow body of water called Sally’s Gut, showed both his potential and his peril.

He’s nowhere in the polls; three months ago he was drawing 6.4 percent of the likely New Hampshire Democratic vote in the Saint Anselm College poll; this month he dropped to 0 percent. But in an hour of give-and-take with New Hampshire voters, he showed a fluency, mastery and nimbleness that he didn’t display in last month’s debate, where he occupied the second-most amount of time and made almost no impression.

Even so, Mr. O’Rourke has top-tier fundraising capacity. He raised an astonishing $6.1 million in the first 24 hours of his campaign and grew that figure to $9.4 million in the next 17 days. Former Gov. George W. Bush of Texas displayed two decades ago the power of a Texas fundraising base for a top-ranked candidate. Mr. O’Rourke, however, raised only $3.6 million in the second quarter.

That drops him far behind the top tier, which comprises Mr. Biden, Mr. Sanders, Sen. Elizabeth Warren of Massachusetts, Ms. Harris and Mayor Pete Buttigieg of South Bend, Indiana.

Mr. O’Rourke entered the race as a matinee idol, the charismatic figure who nearly defeated GOP Sen. Ted Cruz last year but whose defeat hardly diminished his appeal. And when he first ventured here, to the site of the first presidential primary of the 2020 political season, he caused a remarkable stir, repeatedly drawing overflow crowds. Here in Meredith, a few dozen people wandered into the living room of Kate Miller, who lent her home to Mr. O’Rourke without offering her support to him. “What the heck,” she said. “It’s easy to do.”

Many of the visitors, moreover, were vacationers who can’t vote in this state. Some of the people who held Beto signs did it out of politeness, not political commitment. “I was just asked to hold his sign,” said Lee Healey. She is from Glenville, New York. A young campaign aide tried to whip up enthusiasm with a cheerleader chant of “Let’s go, Bet-o,” but the crowd response was dutiful. It faded once he loped into the room.

Yet Mr. O’Rourke possesses a peppiness that few other candidates share, along with a sense of energy as big as Texas; he did six events in the state in less than 48 hours in his mid-month trip here. Some of his lines — “this amazing democracy that’s being tested as never before in our country” — feed the anti-Trump sentiment that is the oxygen of Democrats this year. But others — calling for “the will to change what is happening right now” — seem mushy.

Indeed, Mr. O’Rourke, straddling the top and bottom tiers of candidates, also is straddling the left and center of his party. He’s not for “Medicare for all,” a negative for the progressive crowd. Nor for eliminating the Senate filibuster, another demerit for the left, especially since he essentially filibustered the question itself. But he’s appalled at the Trump administration’s immigration policies, a check mark for the left. And he’s critical of Israel’s treatment of Palestinians, a reassuring bow for the liberals.

But (another lurch to the middle) he’s not for BDS, the boycott, divestment and sanctions movement aimed at Israel. And (here we go with a qualification designed to quell worries) he’s not opposed to other people supporting BDS.

A lot of this needs clarification, but then again, he is not the only candidate trying to have everything both ways. Mr. O’Rourke has little time to achieve that clarity, however. He’s skinny, to be sure, but he has to prove he’s not a trimmer. He’s bright, but he must prove he is not a political comet streaking across the Democratic firmament.

“The early flame has gone out,” said Neil Levesque, who directs the New Hampshire Institute of Politics at Saint Anselm College. “But he can re-light it if he gets the right moment.”

There isn’t much time to reignite that fire. The top tier of candidates is pulling away from the others, and nature abhors a two-dozen-candidate field as much as it abhors a vacuum. A brutal winnowing is imminent.

“It’s happening right now,” says Michael Whouley, who ran Hillary Clinton’s successful 2008 campaign here and was a strategist for Vice President Albert Gore in 2000 and Mr. Kerry in 2008. “As some [candidates] see it’s not going to work for them, they’ll drop out. There’s eventually going to be only one tier. It’s going to become difficult for candidates to break into it.”

Whether Mr. O’Rourke is poised to make that break or to join Rep. Eric Swalwell of California as a dropout is one of the campaign’s pressing questions. The answer may be only a few days away. If he doesn’t step up this week, he may be forced to step away. “Democracy,” Herbert Hoover said after his defeat in 1932, “is a harsh employer.” He, too, began his career in elective politics with great hope and fanfare.

David M. Shribman is a former and now emeritus executive editor of the Post-Gazette and a nationally syndicated columnist.