  We conducted a daily monitoring campaign on Mrk 421 in 2003 and 2004 with the
Whipple 10 m telescope and the large-area instruments aboard RXTE,
simultaneously covering TeV and X-ray energies. Supporting observations at
optical and radio wavelengths were also frequently carried out. Mrk 421 was
observed over a wide range of fluxes (with a dynamic range of ~30 both at TeV
and X-ray energies). The source was relatively quiet in 2003 but became
unusually active in 2004, with flares reaching peak fluxes of ~80 mCrab in
X-rays and >3 Crab at TeV energies! We will describe the multiwavelength
campaign and present some preliminary results. We will also discuss the
implications of the results on the proposed emission models for TeV blazars.
