Editor's note: This story has been updated.

U.S. Rep. Beto O'Rourke, D-El Paso, is sailing toward a 2018 Senate campaign — an uphill battle that would pit the little-known congressman against one of the state's most prominent Republicans in the unpredictable era of President Donald Trump.

"I really want to do this," O'Rourke said in an interview Saturday in which he also promised to run a positive campaign against U.S. Sen. Ted Cruz, R-Texas — no matter how much animus the incumbent inspires among Texas Democrats.

"Being against Ted Cruz is not a strategy," O'Rourke said. "It might motivate some folks and might make the election of a Democrat for the first time in 30 years more likely, but it in itself is not a strategy, and so I'm really putting my time and my efforts and my thinking into what makes Texas a better place and what makes the lives of the people who live in this state better, and so I'm just going to stay focused on that."

O'Rourke has said for weeks that he is likely to take on Cruz but has not set a timeline for an official announcement. He said Saturday he wants to make sure he is mindful of his current constituents and that "I'm thoughtful in how I make this decision and keep El Paso, my family, foremost in mind."

"I don't want to run unless we're going to win, and I'm confident we can," O'Rourke said. "I just want to make sure the way we do this, we set ourselves up for victory."

O'Rourke's case for the Senate seat is two-pronged. He said he believes it is more important than ever for the Senate to serve as a check on the president, and he also believes he brings a unique perspective to the immigration debate as a Democrat from El Paso — "the Ellis Island of the western hemisphere."

O'Rourke may have Democratic company in his campaign to unseat Cruz. U.S. Rep. Joaquin Castro of San Antonio is also weighing a bid and plans to announce his decision in early April, a timeline that O'Rourke said has no bearing on his own.

"I have the greatest respect for him, consider him a good friend and have told him I think he'd make an outstanding candidate and a terrific senator for the state of Texas, but his decision-making process is outside of my control, so I can only focus on what I can do," O'Rourke said.

If it came to it, O'Rourke said he would be open to a contested primary against Castro, again noting that is beyond his control.

If O'Rourke runs for Senate, fundraising would likely be one of his biggest challenges. While he was the underdog in his 2012 Senate campaign, Cruz has since built a national fundraising network, partly through his 2016 presidential bid.

O'Rourke has already made clear he plans not to accept PAC money in a potential Senate campaign. Asked Saturday if that would apply to money from national Democratic groups who may want to help him out, O'Rourke held firm that he "won't take money from political action committees — and that's across the spectrum."

"I think folks just need to know that, clean and simple," O'Rourke said. "When you start picking and choosing then, you know, it becomes a slippery slope and you just start doing what everyone else is doing, what everyone is so sick of and what has made Washington so dysfunctional and corporate."

O'Rourke was visiting Austin on Saturday to speak at a rally at the Texas Capitol against some of Trump's early actions as president, including his proposed border wall. Castro was also scheduled to address the rally. O'Rourke told the crowd that it is a "time for us take back our communities, our state, the United States Senate and the United States of America."

Castro also spoke at the rally, invoking Cruz twice as he denounced Trump's policies. "I hope today that Donald Trump and Ted Cruz can hear us from Washington, D.C," Castro said shortly after taking the stage to loud cheers.

Working the crowd afterward, Castro encountered some supporters who encouraged him to run for Senate — including a man who said he had never donated the maximum amount to a campaign before but said he would do so for Castro.

Speaking with the Tribune after the rally, Castro said he is looking to announce his 2018 decision "by the end of April" and took a pass on responding to recent jabs from Cruz. The incumbent had suggested in a radio interview that Castro would be "retired from public service" if he got into the 2018 race.

"Everything that's going on now is bigger than Ted Cruz," Castro said, "and it's bigger than me, honestly."