U.S. Sen. Marco Rubio is calling for the head of the Internal Revenue Service to resign over the agency’s admitted targeting of conservative groups, including The Tea Party.

Rubio, a Florida Republican and Tea Party favorite, said in a letter Monday to Treasury Secretary Jack Lew: “[I]t is clear the IRS cannot operate with even a shred of the American people’s confidence under the current leadership.”

“I strongly urge that you and President Obama demand the IRS Commissioner’s resignation, effective immediately,” Rubio said in the letter. “No government agency that has behaved in such a manner can possibly instill any faith and respect from the American public.”

In a news conference Monday, Obama said he learned of IRS's focus on conservative groups when news reports broke Friday.

“I will not tolerate it, and I will make sure we find out exactly what happened on this,” Obama said. “That’s outrageous and there's no place for it.”

Sen. Max Baucus said the Senate Finance Committee will investigate the IRS targeting conservative political groups, joining a growing list of congressional panels looking into the matter.

The Finance Committee would be the first in the Democratic-controlled Senate to announce an investigation. The Montana Democrat is the panel's chairman.

In the House, the Ways and Means Committee and the Oversight and Government Reform Committee also are investigating.

The IRS, an independent agency in the Treasury Department, has already apologized for scrutinizing the tax-exempt status of groups with conservative titles such as "Tea Party" or "Patriot" in their names. And White House spokesman Jay Carney said Friday it was wrong.

On Friday, U.S. Sen. Ted Cruz, a Texas Republican who is popular among Tea Party members, released a statement condemning the IRS and asking if it could be trusted with Americans’ personal information.

“What’s just as concerning is the expansive role this agency is charged to take in enforcing Obamacare,” Cruz said. “An agency that has admitted to engaging in such corrupt, political activities has no place assessing or monitoring Americans’ personal health information.”

“According to GAO, the IRS has 47 different taxes and regulations it is expected to administer,” Cruz said, “which not only include collecting and enforcing taxes, but also mining our health care data and monitoring whether Americans’ health plans meet Obamacare thresholds.”

Sen. Susan Collins, R-Maine, said Sunday she was disappointed that Obama "hasn't personally condemned this." The president, Collins said, "needs to make crystal clear that this is totally unacceptable."

Collins and other Republicans challenged the tax agency's claim that the practice was initiated by low-level workers.

"I just don't buy that this was a couple of rogue IRS employees," said Sen. Susan Collins, R-Maine. "After all, groups with 'progressive' in their names were not targeted similarly."

If it were just a small number of employees, she said, "then you would think that the high-level IRS supervisors would have rushed to make this public, fired the employees involved, apologized to the American people and informed Congress. None of that happened in a timely way."

The IRS said Friday that it was sorry for what it called the "inappropriate" targeting of the conservative groups during the 2012 election to see if they were violating their tax-exempt status. The agency blamed low-level employees, saying no high-level officials were aware.

But according to a draft of a watchdog's report obtained Saturday by The Associated Press that seemingly contradicts public statements by the IRS commissioner, senior IRS officials knew agents were targeting tea party groups as early as 2011.

The Treasury Department's inspector general for tax administration is expected to release the results of a nearly yearlong investigation in the coming week.

Lois G. Lerner, who heads the IRS division that oversees tax-exempt organizations, said last week that the practice was initiated by low-level workers in Cincinnati and was not motivated by political bias.

But on June 29, 2011, Lerner learned at a meeting that groups were being targeted, according to the watchdog's report. At the meeting, she was told that groups with "Tea Party," ''Patriot" or "9/12 Project" in their names were being flagged for additional and often burdensome scrutiny, the report says.

The 9/12 Project is a group started by conservative TV personality Glenn Beck.

Rep. Mike Rogers, R-Mich., said "the conclusion that the IRS came to is that they did have agents who were engaged in intimidation of political groups is as dangerous a problem" as the government can have.

He added, "This should send a chill up your spine. ... I don't know where it stops or who is involved."

Congressional Republicans already are conducting several investigations and asked for more.

"This mea culpa is not an honest one," said Rep. Darrell Issa, R-Calif.

After the Associated Press report, Carney said that if the inspector general "finds that there were any rules broken or that conduct of government officials did not meet the standards required of them, the president expects that swift and appropriate steps will be taken to address any misconduct."

Collins said the revelations about the nation's tax agency only contribute to "the profound distrust that the American people have in government. It is absolutely chilling that the IRS was singling out conservative groups for extra review."

The IRS' Lerner said that about 300 groups were singled out for additional review, with about one-quarter scrutinized because they had "tea party" or "patriot" somewhere in their applications.

She said 150 of the cases have been closed and no group had its tax-exempt status revoked, though some withdrew their applications.

Collins appeared on CNN's "State of the Union," Rogers was on "Fox News Sunday" and Issa spoke on NBC's "Meet the Press."

The Associated Press contributed to this story.

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