The Democratic and Republican senators pitching an infrastructure deal face early hurdles in pushing their roughly $1 trillion plan through Congress.The bipartisan proposal crafted by 10 senators would focus on transportation, broadband and water, and would not raise taxes to offset costs. A handful of Democrats who seek a broader plan that addresses climate change and social programs, paid for by increasing taxes on corporations or the wealthy, have opposed the framework.Senators have to walk a fine line as concessions to win over one party jeopardizes support from the other. Despite growing opposition from liberals, one Republican who worked on the plan hopes the group can gain support from enough GOP senators to overcome a loss of Democratic votes.”There certainly should be,” Sen. Rob Portman, R-Ohio, told CNBC on Tuesday when asked whether there would be enough Republican support to pass the plan. “I mean, this is a proposal for infrastructure that Republicans have traditionally supported. It’s also a proposal without raising income taxes. … I think it’s something that’s going to get a lot of support on both sides of the aisle.”CNBC PoliticsRead more of CNBC’s politics coverage:President Joe Biden proposed a $2.3 trillion infrastructure and economic recovery plan as his second major legislative initiative. After his talks with Republicans collapsed amid disagreements over what to include in a bill and how to pay for it, lawmakers pushed ahead with a last-ditch effort to craft a bipartisan plan.As the 10 senators try to win support for their proposal, Democrats have set the groundwork to approve a bill on their own through budget reconciliation. However, the path appears blocked for now as at least one Democrat involved in the talks, Joe Manchin of West Virginia, insists he wants to pass a plan with bipartisan support.Congressional leaders have a math problem. To get through the evenly split Senate under the normal process, legislation would need support from all of the Democratic caucus and at least 10 Republicans — or more if any Democrats defect. If Democrats try to approve legislation on their own using budget reconciliation, they cannot lose a single vote.U.S. Senators Mitt Romney, Kyrsten Sinema, Susan Collins, Joe Manchin and Mark Warner depart after attending a bipartisan work group meeting on an infrastructure bill at the U.S. Capitol in Washington, U.S., June 8, 2021.Evelyn Hockstein | ReutersThe bipartisan strategy faces its share of skeptics. Sen. Bernie Sanders, a Vermont independent who caucuses with Democrats, told reporters Monday he will not vote for the plan.”The bottom line is, there are a lot of needs facing this country,” he said. “Now is the time to address those needs, and it has to be paid for in a progressive way, given the fact that we have massive income and wealth inequality in America.”At least two other Democrats — Sens. Ed Markey of Massachusetts and Jeff Merkley of Oregon — have signaled they will oppose an infrastructure deal unless it invests more in fighting climate change.Passing a bill in the Senate will also depend on whether the bipartisan group can win over Majority Leader Chuck Schumer, D-N.Y., and Minority Leader Mitch McConnell, R-Ky. Neither senator has endorsed the framework.McConnell is “open minded, as he’s said to the media. … I think Democrats are talking to Sen. Schumer as well, and I think he’s also open minded,” Portman told CNBC.While McConnell has said he hopes to reach a bipartisan infrastructure deal, he has also vowed to fight Biden’s economic agenda.Schumer said Monday that “discussions about infrastructure investments are progressing on two tracks.” The Democrat added that as bipartisan talks take place, Senate committees are also working on a plan based around Biden’s proposal, “which will be considered even if it does not have bipartisan support.”He also signaled he wants to see a larger investment in climate action.”And as a reminder to the Senate, a reminder to the Senate: as I’ve said from the start that in order to move forward on infrastructure, we must include bold action on climate,” he said.The challenges are not limited to the Senate. House progressives have started to come out against a bipartisan plan smaller than the one Biden proposed. House Speaker Nancy Pelosi, D-Calif., also said a provision to index the gas tax to inflation would not get the White House’s blessing.”The president of the United States is a major factor in this, and he has said he would not support any taxes on people making under $400,000 a year, and that includes increasing the gas tax,” she told CNN on Sunday.Portman said Tuesday that the bipartisan framework would include a “slight increase” to the tax.Pelosi on Sunday did not rule out her caucus supporting a more narrow infrastructure package. She said Democrats would likely need assurances they will next pass a broader bill that includes more party priorities.”If [a bipartisan deal] is something that can be agreed upon, I don’t know how we can possibly sell it to our caucus unless we know there is more to come,” she said.Subscribe to CNBC on YouTube.