History books will show that Barack Obama became president of the United States, taking the oath of office, on January 20, 2009.

But Obama didn't really become leader of the world's most powerful nation until sometime earlier this month, after winning a bruising political battle over health care reform.

Since that victory, Obama has governed with a renewed vigor and no longer seems shackled by his earlier pledges to work with Republicans in Congress.

In other words, Mr. Nice Guy has left the White House.

Gone is the timid, weak Obama, who seemed to treasure negotiating with stubborn Republicans above getting his own agenda passed. On a host of topics, including presidential appointments and education funding, Obama has pressed forward in the face of Republican fussing.

My guess is that the partisan battle over health care reform is what showed Obama that trying to work with Republicans would be a waste of time in most cases.

Obama spent more than a year trying to craft health care reform to Republican tastes. He even removed the "public option" provision cherished by progressives of his own party. When Obama hosted his pivotal health care summit with GOP leaders, his request for substantive ideas on how to craft the law was met with demands to scrap the reform package and start all over again.

Maybe that's when the lightbulb in Obama's head switched on. It sure did for me.

Republicans are spending every minute plotting his defeat and won't put the interest of voters above politics for a second. So forget about negotiating with them.

With both the House and Senate in Democratic hands (at least until November's elections), this may be the last chance to push his policies with the support of a friendly Congress.

So when it came time this week to push his proposal to make higher education more affordable, Obama didn't spend time negotiating with Republicans. Instead, he signed the law that will save taxpayers millions by removing banks from acting as middlemen in federally guaranteed student loans.

Flexing more presidential muscle, Obama used his constitutional authority and made two recess appointments on the National Labor Relations Board.

Both moves were bitterly opposed by Congressional Republicans.

Obama's new aggressive stance will have its drawbacks, especially when he is pushing a policy I disagree with.

For example, Obama announced Tuesday he is taking a page from the Republican playbook to allow oil drilling 50 miles off the Virginia shoreline.

Environmentalists in the Democratic Party will howl, but they will learn a lesson Congressional Republicans are coming to grips with:

Mr. Nice Guy has left the White House.