Laid off while on maternity leave because employer suppossedly could not afford me, however the temp was kept.  Can I sue? 

You may have a claim here.  Federal law protects pregnant workers and job applicants from discrimination.  Under Title VII, employers may not refuse to hire/fire a woman because she is pregnant, take away benefits (including accrued seniority) because a woman takes maternity leave, or take away benefits from a single mother.  Generally, an employer must treat pregnant women the same as other workers who cannot perform their jobs for short periods of time. Accordingly, if an employer allows employees to take leave for a broken arm or short-term illness, the employer must allow pregnant women to take the same leave.

Pregnancy leave also is protected under the Family and Medical Leave Act ("FMLA").  FMLA allows employees to take up to 12 weeks of unpaid leave to attend to family matters.  A person must be qualified to do so.  To qualify an individual must be a full-time government employee, or employee of a company with 50 or more employees, who has worked for the company at least 12 months and at least 1,250 hours during the 12 months immediately prior to taking leave.  When an employee returns from leave, they are entitled  to the same job that they left.  If the same job is not available, the employer must place the employee in an equivalent job with equivalent pay, benefits, and duties.

Additionally, state laws may apply to your situation as well.  What you need to do now is to speak with an employment attorney in your area.