• An unpaid-for standalone military aid package to Israel could be on the table, a Republican senator said on CNBC's "Squawk Box."
• But Congress and the White House must hash out a deal to beef up U.S. border security first, said Sen. Roger Marshall, R-Kan.
• Otherwise Republicans will oppose efforts to pass aid for "Ukraine, for Taiwan, as well as for funding for Israel," he said.

A standalone military aid package for Israel, with no cuts to offset it, is on the table, a Republican senator said Monday. But first, Congress and the White House must hash out a deal to beef up U.S. border security.

"I don't care how we pay for it," Sen. Roger Marshall, R-Kan., said of the Israel aid in an appearance on CNBC's "Squawk Box."

"I'd even consider standalone legislation just support Israel without a pay-for, I think it's that big of a priority," said Marshall, a member of the Senate Budget and Homeland Security Committees.

But he said funding Israel's war against Hamas is just one piece of a complicated riddle, in which Senate Republicans want a bipartisan border security package that grants the president power to shut the border, in exchange for seriously considering a White House request for emergency foreign aid to Israel, Ukraine and Taiwan.

"The rest of the riddle is not going to be solved unless we have meaningful border security," Marshall said.

"If that riddle is not solved, then the Republicans are going to vote down any type of cloture for the other three pieces of this riddle, for Ukraine, for Taiwan, as well as for funding for Israel."

The Senate hopes to finalize this deal by the end of the year, a lead Republican negotiator, Sen. James Lankford, Okla., said over the weekend.

But this timeline could potentially be disastrous for Ukraine, the White House warned Monday.

Unless Congress acts, the U.S. will run out of money to supply Ukraine more weapons and equipment by the end of the year, Office of Management and Budget Director Shalanda Young wrote in a letter to House Speaker Mike Johnson, R-La.

Shutting the flow of resources to Ukraine "will kneecap" its military efforts, threatening Kyiv's battlefield gains and increasing the likelihood of Russian military success, Young wrote.

"If Ukraine's economy collapses, they will not be able to keep fighting, full stop," Young wrote.

"We are out of money — and nearly out of time."

Johnson responded later Monday that the White House is failing to address House Republicans' concerns about further funding Ukraine while "continually ignoring the catastrophe at our own border."

"House Republicans have resolved that any national security supplemental package must begin with our own border. We believe both issues can be agreed upon if Senate Democrats and the White House will negotiate reasonably," Johnson said on X.