Editor's note: This story has been updated.

OTTUMWA, Iowa — The questions came almost immediately, as they usually do.

Do you think you think you'll be picked as her running mate? No. 

Are you auditioning to be vice president? No.

Do you want to be vice president? No.

It has become a familiar routine for U.S. Housing Secretary Julián Castro, the former mayor of San Antonio long rumored to be on Hillary Clinton's shortlist for running mate. But it is finding a wider audience as Castro steps up his involvement in the campaign with the first nominating contests set to begin in eight days — and Clinton flooding this state with some of her top surrogates.

Arriving Sunday morning at a Clinton field office in this southeastern Iowa city, Castro could barely make it across the room before the topic came up — courtesy of a Clinton volunteer who admiringly greeted him as "Mr. Vice President." Speaking with reporters after making his pitch for Clinton, Castro did not say anything new about the vice presidential speculation — though it was not for a lack of prodding. 

"I'm focused completely on being supportive of Hillary. There's a Democratic primary," Castro said, reiterating his belief Clinton ultimately will not pick him for the job. "I don't believe that that's going to happen, and I've said that."

"I fully expect to be back in Texas a year from now," Castro added, using a line that earned some gentle pushback from an audience member during a stop two hours later in Fairfield.

“I hope you’re not watching from San Antonio," said Holly Moore, a local artist who volunteers for the campaign. "I hope you’re somewhere else. A lot of people in this room feel that way."

Castro's prospects got a boost Saturday in his hometown, where Javier Palomarez, the head of the U.S. Hispanic Chamber of Commerce, made an unprecedented endorsement of Castro for the No. 2 spot on the ticket. Asked what that tells him about the enthusiasm for him as Clinton's potential vice president, Castro took another pass on wading into the speculation. 

The routine played out at the first stop of four Castro made Sunday across eastern Iowa,working his way from Ottumwa to Davenport. The Clinton campaign has dispatched him and his twin brother, U.S. Rep. Joaquin Castro of San Antonio, to a number of early voting states as the race continues to tighten between Clinton and U.S. Sen. Bernie Sanders, an Independent from Vermont.

Rallying groups of volunteers at each stop — some of them only taking brief breaks from making calls to hear him speak — Castro urged them to go above and beyond in anticipation of a close finish. At more than one stop, he invoked the 2000 presidential race, when Republican George W. Bush ultimately beat Democrat Al Gore thanks to a razor-thin margin in Florida, to argue elections matter — and emphasize the stakes of the current contest.

“We absolutely can’t afford to hand over the presidency to the Republican Party," Castro said. "Can you imagine what would happen if we have Speaker Ryan, Senate Majority Leader McConnell and President Trump or President Cruz or any of these folks?”

Among the last times a Castro brother was in Iowa, it was Joaquin — and he ended a day of events for Clinton with a sharp critique of Sanders' ability to appeal to the Latino community. On Sunday, Julián Castro responded to a question on the topic by zeroing in on Sanders' health care plan, which he said would undermine President Barack Obama's signature legislative achievement. 

"Latinos have been the biggest beneficiary of the Affordable Care Act, so the idea that we would open that back up to a Republican Congress and give them the opportunity to eviscerate it does not make me feel comfortable," Castro told reporters. "I would say that Bernie is talking about opening that back up and putting it at the whim of the Republican Congress.

"I believe that's the completely wrong way to go," Castro added. "I believe that we should build on the Affordable Care Act, and the Latino community has benefited so much from that I don't want to risk those gains."

It was not the only time Sanders came up Sunday. During a visit in the afternoon to another Clinton office, Castro argued Republicans are mischievously boosting Sanders because they would rather face him as the Democratic nominee than they would Clinton.

"I saw them tweeting out supportive tweets the other day for another candidate," Castro said. "It’s amazing because they know that Hillary will fight for the people that they too often ignore and they don’t represent, and that if she gets in to the general election that she’s going to win in November.”

Regardless of Castro's vice presidential fate, Iowans who saw him Sunday said they were most struck by his touching life story and how it could help Democrats reach Latinos. At each stop, Castro gave a brief overview of his biography, most poignantly recalling his Mexican-born grandmother's pride at seeing him and his brother get accepted to Stanford University.

"What I hear from him just makes me want to weep, and it connects with not only me as someone who has no similarity to him at all in features or in background," said Lois Abel-Priester, a Clinton volunteer from Bloomfield who runs an education consulting firm.

Moore, the volunteer in Fairfield, later said she could not think of any other Democrat able to fill out the ticket as well as Castro, whom she called "freakishly qualified on so many levels."

"I don't entertain it as seriously because I've found my candidate," she said of considering other potential running mates, quickly clarifying. "My vice presidential candidate."