Portman, addressing a small group of reporters just after announcing that he will leave the administration this summer, said Bush will veto bills that contain excessive spending, and Congress will have to decide whether it wants to shut down the government over the matter.
"He's going to stick to the vetoes. He would very much like to avoid a government shutdown," Portman said, but "that will be a decision Congress has to make." He noted Congress could also pass a continuing resolution to maintain government operations.
The last government shutdown occurred in 1995, when the new Republican Congress and President Clinton could not resolve a standoff. Since then, lawmakers have used continuing resolutions to keep the government running while spending differences were resolved.
Portman noted that the administration has sought to make clear its objections to spending early in the process, suggesting that Democrats have had fair warning about Bush's budget requirements.
President Bush announced Tuesday that he had chosen former House Budget Chairman Jim Nussle, an Iowa Republican, to replace Portman. With or without a showdown with Congress over this year's spending bills, Portman's successor faces what is likely to be a bitter battle over the fiscal 2009 budget early next year. With the White House and control of Congress at stake in next year's elections, the political rhetoric is expected to be particularly heated.
While Portman said Bush would stand his ground, he did say the president would be flexible on how money is appropriated, as long as the total is within his overall discretionary spending limit.
But that also means early Democratic increases in the veterans spending bill, which Bush appears likely to swallow, will have to be paid for elsewhere.
"We're not going to try to micromanage the process," Portman said. "If Congress can come up with a top line of $933 [billion] -- which is a 6.9 percent increase from 2007, which is triple inflation or more -- then we will be happy to work with them. But until we see a path to the top line, we will be forced to veto many of these bills as they come through the process."
Portman, who said he is leaving the White House to spend more time with his family, was not shy about saying he is considering a run for governor of Ohio in 2010.
Portman dismissed suggestions that his association with an unpopular president and his pursuit as Bush's trade representative of open markets would harm him in Ohio, where widespread concerns about open trade helped sweep Democratic Sen. Sherrod Brown into office last year.
"Ohio's economically dependent on trade," Portman said. "I welcome that debate." He said his 12 years in Congress probably have more relevance for Ohio voters than his years with Bush. "It matters what your record is and what you've done," he said, though adding he is "proud" of his service under the president.
Portman, known for being able to work with Democrats, said Nussle would be able to pick up where he left off.