It's easy to say Scott Walker dropped out of the Republican presidential race because he ran out of money. But he ran out of money because of his own limitations as a candidate — limitations that have been in plain sight for quite a while.

Walker surprised the political world in late January when he delivered a wow-'em speech at an Iowa Republican gathering. The GOP activists there already respected Walker for his record as governor of Wisconsin, but most expected a lackluster, even boring presentation from a man who had not, up to that point, set the campaign trail on fire.

What they got was a strong and stirring speech that made a lot of Republicans in the crowd immediately rethink their assessment of Walker. If he could combine a great record with a dynamic candidacy -- well, Scott Walker could be The Man.

Walker shot up in the polls. But signs of trouble quickly followed.

There had always been talk that Walker, as a Midwestern governor, wasn't well versed, or even very versed at all, in foreign policy. That turned out to be true, and obvious to all when he cited his command of the Wisconsin National Guard as national security experience and argued that Ronald Reagan's 1981 firing of the air traffic controllers was "the most significant foreign policy decision of my lifetime."

Some supporters saw Walker's lack of foreign policy chops as a fixable problem. Indeed, he tried to fix it, gathering a group of experts to school him in international affairs. But for Walker, an even bigger problem was domestic policy. He just wasn't very up on some of the key policy and political issues that a president has to confront.

About a month after his Iowa breakthrough, Walker traveled to Palm Beach, Florida to address a donor-heavy crowd at a gathering sponsored by the conservative Club for Growth. He was asked his thoughts about the Export-Import Bank -- not a huge issue, but an important one to many fiscal conservatives -- and he didn't seem to have any. Walker was also asked about the standoff then going on in Congress over funding the Department of Homeland Security. His answer was long, meandering, and entirely unclear. He was asked about President Obama's executive action on immigration. Same story.

Walker was not a candidate prepared to deal with national policy in the context of a presidential campaign. In an interview, I asked him whether things had moved too quickly, whether the ground had shifted under his feet after the Iowa speech. His answer was instant: "Totally."

"We thought all along if we got in, it would be kind of this slow and steady, don't worry about the other guys, just keep focused on moving forward, and as candidates chose not to get in or fell off, we'd be in a position to make a compelling case to them," Walker explained. "We had no idea that after that Iowa summit there would be that kind of acceleration to the race. But we're here, and we're not going to complain about it."

Still, many Walker supporters thought the problems were fixable. So did Walker. He could get those experts together, dive into the briefing books, and find his footing.

It didn't work. As the campaign went on, Walker made error after error, all based in the fact that he wasn't well versed in national issues. When he took multiple positions on the question of birthright citizenship -- again, not the biggest issue in the world but the kind of thing that can pop up in a campaign -- it was clear he hadn't really thought about it very much.

The hard lesson for Walker is that campaigns expose a candidate's weaknesses and gaps in knowledge. While it is possible for candidates to improve as performers -- they do it all the time -- it is really hard for them to learn much new during the campaign. The action is simply too frantic, too non-stop for a candidate to really delve into anything.

What that means is a good candidate had better bring a pretty strong store of knowledge to a campaign. Walker brought a lot of knowledge about Wisconsin, but not a lot about presidential-level issues.

His deficiencies were brutally exposed once the GOP debates began. At the Fox News debate in August, Walker went in too cautiously -- the idea was to hit a single, avoid any big mistakes, and move on -- and underperformed even those expectations.

Going into the CNN Reagan Library debate last Wednesday, Walker's team had done a lot of coaching, but mostly, it seemed, on style. He talked too fast in the first debate, so they wanted him to write two words on a pad of paper when he arrived at his podium at the Reagan Library: "Slow Down."

It didn't matter. The moderators virtually ignored Walker, and he failed to effectively jump into the stream of debate on his own. Then, after the debate, came a shocking poll, from CNN, that showed Walker's support, already weakening, had cratered.

Many Republicans will view Walker's departure with real regret. It is hard to overstate how much conservatives respected him for his fight against the public employee unions in Wisconsin. He stood up to everything the Democratic-Big Labor-Liberal Establishment had to throw at him, and came out the winner. That took enormous strength and resolve. If anything qualified a Republican for a shot at the White House, that was it.

But for all this strengths at the state level, Walker just wasn't ready for a national run. And in the end, the presidential campaign did what presidential campaigns do: it ruthlessly exposed the weakness of the man at the top.

Walker bows out in Politics on LockerDome