The Securities and Exchange Commission is moving toward charging former and current Fannie Mae and Freddie Mac executives with violations related to the financial crisis, setting up a clash with the housing regulator that oversees the companies, according to sources familiar with the matter. The SEC, responsible for enforcing securities laws, is alleging that at least four senior executives failed to provide necessary information to investors about the companies’ mortgage holdings as the U.S. housing market collapsed. But the agency that most closely regulates Fannie and Freddie, the Federal Housing Finance Agency, disagrees with that assessment, according to sources familiar with the matter.