Baucus said Trump's speech on Thursday was good and the G-20 is an opportunity for the president to show "American strength" and gain the respect of allies.
"I hope he is well prepared going at least into the bilateral with President Putin," Baucus said. "If he does a really good job, sticks with the messages the United States has, then he will earn much greater respect."
Hours before the speech, Trump downplayed the conclusion by some top U.S. intelligence agencies that Russian interfered in the election, saying it may have but other countries may have meddled, too.
"Nobody really knows. Nobody really knows for sure," Trump said.
The president's remark came as a special counsel investigates Russian efforts to influence the election and possible ties between the Trump campaign and the Kremlin.
— CNBC's Jacob Pramuk contributed to this report.